This invention relates to vacuum switches in general and more particularly to an improved vacuum switch of the type having an insulating strut.
A vacuum switch with a head piece held by an insulating support and a base held by a further insulating support, and including a switching tube arranged between the head piece and the base and at least one insulating strut connecting the head piece and the base, which has a transverse hole for receiving fastening means at each end, is described in Siemens-Zeitschrift 51 (1977), No. 4, pages 278 to 281. An insulating strut of this type is used, for instance, in vacuum circuit breakers for the medium voltage range in order to stiffen the mounting of the vacuum switching tubes consisting of insulating supports against the stresses occurring in the switching process. It has been found sufficient and practical to make the insulating struts as smooth rods, i.e. without ribs. Such rods can advantageously be made of insulating material in sheet form by subdivision. They are inexpensive and have the desired high tensile strength. However, it has been found on occasion that these insulating strusts do not fully meet the requirements with respect to dielectric strength, especially if they are used in switchgear in the upper range of the medium voltage.
As a remedy it would be possible to consider making the insulating struts not smooth, as heretofore, but with ribs similar to conventional support insulators, or in another shape which extends the leakage path. This, however, would result in considerable additional cost. For, the insulating struts would have to be made in a mold as individual parts while they have heretofore been produced by subdividing insulating material in sheet form. In addition, the high tensile strength of the present insulating struts cannot be obtained without difficulty with molded or injection molded parts.
It is known, on the other hand, that the insulating capacity of supports in medium and high voltage equipment can be increased by so-called grading electrodes which cause the influence of the fastening parts on the dielectric strength to be decreased or made ineffective (German Pat. No. 853 018; Swiss Pat. No. 279 664). It is an object of the present invention to equip the insulating struts with such grading electrodes in as simple a manner as possible.